This is the HPI One-Way Bearing for the HPI F4.1 (which is the 4.1 engine on the Savage X), the Nitro Star BB Series & Force 21 & 25, and G3.0 engines from HPI, as well as the Force 21 & 25 from Ofna. This fits the Savage X 4.1 as well as the.21 and .25 Savage engines. This one-way bearing allows the recoil starter to grab the crankshaft to start the engine and spin freely once running.